Two-dimensional position sensors may be based on capacitive proximity sensing techniques. Such sensors may be referred to as 2-dimensional capacitive transducing (2DCT) sensors. 2DCT sensors may be based on detecting a disturbance in a capacitive coupling of sensor electrodes caused by the proximity of a pointing object. A measured location for the disturbance corresponds to a measured position for the pointing object.
2DCT sensors are typically actuated by a human finger, or a stylus. Example devices include touch screen and touch sensitive keyboards/keypads, e.g. as used for controlling consumer electronic devices/domestic appliances, and possibly in conjunction with an underlying display, such as a liquid crystal display (LCD), or cathode ray tube (CRT). Other devices which may incorporate 2DCT sensors include pen-input tablets and encoders used in machinery for feedback control purposes, for example. 2DCT sensors are capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part, by means of a capacitance sensing mechanism.
Devices employing 2DCT sensors have become increasingly popular and common, not only in conjunction with personal computers, but also in all manner of other appliances such as personal digital assistants (PDAs), point of sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like. 2DCT sensors are frequently preferred to mechanical switches for a number of reasons. For example, 2DCT sensors require no moving parts and so are less prone to wear than their mechanical counterparts. 2DCT sensors can also be made in relatively small sizes so that correspondingly small, and tightly packed keypad arrays can be provided. Furthermore, 2DCT sensors can be provided beneath an environmentally sealed outer surface/cover panel. This makes their use in wet environments, or where there is a danger of dirt or fluids entering a device being controlled attractive. Manufacturers often prefer to employ interfaces based on 2DCT sensors in their products because such interfaces are often considered by consumers to be more aesthetically pleasing than conventional mechanical input mechanisms (e.g. push-buttons).
One prior 2DCT sensor includes a substrate with a sensitive area defined by a pattern of electrodes. The 2DCT may be of the so-called “active” or “mutual” type, in which proximity of an object is sensed by the changes induced in coupling between a drive electrode and one or more adjacent sense electrodes. Measurement of the coupling is carried out by applying a transient voltage to the drive electrode and making a measurement of the capacitance between the drive and associated sense electrode(s) that results.
The pattern of electrodes may include longitudinal (bar) drive electrodes and sense electrodes arranged in an interleaved arrangement between adjacent drive electrodes. The sense electrode pattern comprises four groups of sense electrodes. The groups of sense electrodes co-extend longitudinally having complementary tapers over their distance of co-extension to provide ratiometric capacitive signals. The different regions of co-extending sense electrodes provide ratiometric capacitive signals indicative of capacitive coupling of a user's finger on a part of the sensor where sense electrodes are present. Thus, a user's finger approaching the sensor is sensed by two different electrode groups to provide a beneficial mixing of signals which may be used to determine the x-position of a finger or other object on the sensor. The position of an object on the sensor may be determined by the disruption or reduction of capacitive coupling between a drive electrode and one or more sense electrodes. The signals from the sense electrodes are processed to calculate finger position.
However, it has been found that there are some limitations associated with 2DCT sensors. For example, 2DCT sensors can be sensitive to external ground loading. Furthermore, electrical noise generated from LCD screens can interfere with capacitance measurements when a pointing object approaches the screen. Known methods to minimise the effects of noise on capacitive coupling is to increase the separation or air gap between an LCD screen and an overlaying 2DCT sensor. Alternatively a shielding layer may be incorporated between the LCD screen and a 2DCT sensor to reduce or block the noise induced by the LCD screen.
In one prior device, a capacitive touch sensor has a dielectric panel overlying a drive electrodes with two sense electrodes. A first sense electrode Y0 is positioned to be shielded from the drive electrodes X0, X1, X2, X3 by a second sense electrode Y1, so that the first sense electrode Y0 receives the majority of the charge coupled from the drive electrodes X0, X1, X2, X3 and the second sense electrode Y1 primarily registers noise. A sensing circuit includes two detector channels S0/Y0, S1/Y1 connected to the first (coupled) and second (noise) sense electrodes Y0, Y1 to receive signal samples respectively. The sensing circuit is operable to output a final signal obtained by subtracting the second signal sample from the first signal sample to cancel noise on an output channel.
A further prior capacitive touch sensor has a display device with a touch sensor arranged so that the two dimensional touch sensor is overlaid upon a display panel to form a touch sensitive display screen. The display panel uses an LCD arrangement with vertical and horizontal switching of the LCD pixels driven by a drive circuit. A touch sensing circuit includes a current detection circuit, a noise elimination circuit and a sampling circuit for each of a plurality of sensors, which are arranged to form the two-dimensional sensor array. The current detection circuit receives a strobe signal, which is generated from the horizontal and vertical switching signals of the LCD screen. The strobe signal is used to trigger a blanking of the current detection circuit during a period in which the horizontal switching voltage signal may affect the measurements performed by the detection circuit.
In a further prior capacitive touch sensor device, a two dimensional touch sensor is overlaid on a liquid crystal display (LCD) screen. The effects of switching noise on the detection of an object caused by a common voltage signal of the LCD screen may be reduced by forming the sensor as a plurality of keys. The sensor further includes a capacitance measurement circuit operable to measure the capacitance of the sensing element and a controller circuit to control charging cycles of the capacitance measurement circuit. The controller circuit is configured to produce charging cycles at a predetermined time and in a synchronous manner with a noise signal. For example, the charge-transfer cycles or ‘bursts’ may be performed during certain stages of the noise output signal from the display screen, e.g. at stages where noise does not significantly affect the capacitance measurements performed. Thus, the sensor can be arranged to effectively pick up the noise output from a display screen and automatically synchronise the charge-transfer bursts to occur during stages of the noise output cycle.
However, noise reduction techniques such as those described above require more complex measurement circuitry. This makes the measurement circuitry more expensive and the time taken to complete an acquisition cycle may be increased.